Social blogging platform Tumblr, which surpassed Wordpress.com earlier this year in number of blogs hosted, has raised $85 million in its latest round of funding led by Greylock Partners and Insight Venture Partners, as well as Virgin Group Chairman Richard Branson.

The New York-based start-up also received funds from Union Square Ventures, Spark Capital and Sequoia Capital. With this latest boost, Tumblr has raised more than $125 million.

Founder David Karp said the extra money allows us to continue to scale our business and give real focus to the further development of Tumblr. The site already reports monthly page views of 13 billion.

Tumblr's free and easy-to-use platform has attracted celebrity bloggers like John Mayer, Katy Perry and Lady Gaga, and experienced an enormous spike in popularity over the last year: Tumblr grew 183 percent in unique users from May 2010 to May 2011, and the site celebrated its 10 billionth post in September. In June, the service set a new daily high of 400 million page views.

Tumblr has more pageviews than Wikipedia and Twitter, and is about half the size of AOL and Craigslist, according to ComScore.

At Greylock we see some of the same early dynamics in Tumblr that led us to invest in Facebook, Pandora, and LinkedIn, and so are very excited to add Tumblr to that list, said John Lilly, a partner with Greylock.

In the company's previous round of funding in December, Tumblr raised $30 million from Sequoia Capital, Spark Capital, and Union Square Ventures, earning a valuation of roughly $120 million.

Facebook may still be the dominant social network, but Tumblr still captures a reasonable amount of screen time as well. Tumblr's 30 million blogs generate more than 40 million posts per day, or about 26,000 posts every minute.

Questions around monetization still exist, said Roelof Botha, a Tumblr director and partner with Sequoia Capital. But the company has absolute scale, massive traction, and huge growth and money-making potential.

Tumblr makes its money in advertising, as well as offering a slew of custom blog themes ranging in price from $9 to $49. While the site is not a big cash cow, many other popular online services without lucrative business models have gone on to win big with investors. This year alone, location-based mobile platform Foursquare raised about $50 million in June and earned a $600 million valuation, while perennial IPOer Groupon raised $950 million in January, earning a valuation of $4.75 billion.

However, Tumblr has found success by appealing to the younger demographic. The site has attracted more female teens to the site than any other social network, and 57 percent of Tumblr users are under the age of 34, according to Nielsen..

Tumblr, which combines elements of traditional blogging and Twitter, allows users to share text, photos, links, slideshows, and videos with each other, and also follow and re-post content from each other's blogs.

In June, Tumblr passed Wordpress.com to claim the blog hosting crown with over 21 million hosted blogs. Since that time, the platform has attracted another 9 million blogs.